Digital speech coding in a telecommunications environment using adaptive differential pulse code modulation (ADPCM) is known. ADPCM is a data compression/decompression routine used to exchange voice or data between a high speed data link (e.g., a T1 span line) and a lower capacity data link (e.g., an air interface between a cellular radiotelephone and a cellular base station).
ADPCM has been standardized by the International Telegraph and Telephone Consultative Committee (CCITT) under CCITT Recommendation G.721 for digital speech coding in a telecommunications environment. The standard, as defined by the CCITT, specifies the translation of .mu.-law or A-law pulse code modulated (PCM) encoded speech at 64 kbit/s to ADPCM encoded speech at 32 kbit/s to provide a 2 to 1 compression of the speech signal with very little perceptual loss of speech quality.
ADPCM encoders and decoders take advantage of a tendency of speech signals to vary slowly over time to provide an adaptive step size and compression factor. An ADPCM encoder uses only a coded ADPCM signal for feedback to a prediction and adaptation section. The ADPCM decoder also uses the encoded ADPCM signal for prediction and adaptation. The use of the same feedback parameter results in no need for the transmission of update parameters over the communication channel between encoder and decoder. The use of the same parameter for prediction and adaptation provides two key benefits: 1) the encoder and decoder are almost identical in function, and 2) the encoder and decoder will (in the absence of transmission errors) always converge to the same state with identical internal values.
In order that an ADPCM encoder and decoder be maintained in an identical state (i.e., maintained in convergence), the prior art has taught that a communication channel, once established under ADPCM, must be maintained for the duration of a communication transaction. The prior art has taught that interruption of the communication channel results in divergence of encoder and decoder. At the end of the interruption the encoder and decoder would have to re-converge, which would result in speech interruptions.
Maintaining a channel during speech pauses (or maintaining a second half of a duplex channel while one party is listening) is inefficient in that at least power is unnecessarily consumed. Because of the importance of communications, a need exists for a means of interrupting ADPCM transmissions during normal conversation without losing convergence of encoder and decoder.